1. Field of the Invention
The present invention relates generally to printed circuit boards. More particularly, the present invention relates to printed circuit boards having surface mount devices with improved solder joints.
2. Disclosure Information
In the field of surface mount printed circuit boards (PCBs), an important indicator of solder joint durability is the time required for a solder joint of a surface mount device (SMD) to fail under given conditions of cyclic temperature variation. FIG. 1A shows a typical surface mount PCB having an SMD 14 joined to a mounting pad 12 on a PCB substrate 10 by a conventional solder joint 24. The solder joint 24 has an inner fillet 26 adjacent the device's bottom surface 18 and the mounting pad 12, and an outer fillet 28 extending between the device's perimeter wall 20 and the mounting pad 12. As FIG. 1A illustrates, conventional solder joint inner and outer fillets are concave in shape.
When a solder joint fails, two successive phases of joint failure occur: crack initiation and crack propagation. Crack initiation time is the time required for a crack to first form in the solder joint. As FIG. 1B illustrates, crack initiation generally begins on the inner fillet surface 27. Crack propagation time, on the other hand, is the time from crack initiation until the solder joint fails electrically. The propagation phase consists of two stages: propagation of the crack under the device end termination 22, as shown in FIG. 1C, and propagation in the outer fillet 28, as shown in FIG. 1D. Crack propagation in the outer fillet typically occurs along a line that makes a 45 degree angle with the horizontal extending from the bottom edge 21 of the device up to the outer surface 29 of the fillet. A solder joint fails electrically when the crack propagates to the outer fillet outer surface 29 or substantially thereto, as illustrated in FIG. 1E, such that electrical continuity is functionally broken between the termination 22 and its associated mounting pad 12.
It has been demonstrated that solder joint life is affected by three aspects of solder joint geometry: (1) solder joint height h.sub.o, defined as the vertical distance between the device's bottom terminations and the vertically adjacent mounting pads, as shown in FIG. lA; (2) solder joint inner fillet shape; and (3) solder joint outer fillet shape. Crack initiation time tends to increase with increased solder joint height h.sub.o and with appropriately designed inner fillet shape (i.e., with the inner fillet angle .alpha. being greater than a certain minimum number of degrees). Crack propagation time under the terminations (i.e., stage I) tends to increase with increased solder joint height. It also tends to increase with increased distance between the inner fillet surface 27 and the bottom edge 21 of the terminations; however, this distance is determined by the geometry of the device's terminations and is fixed for a given device. As for crack propagation time in the outer fillet (i.e., stage II), this tends to increase with appropriate outer fillet shape, particularly where the shape requires that the crack propagate a longer distance. Thus, overall solder joint life can be improved generally by increasing the solder joint height and by appropriately designing the shape of the inner and outer fillets.
One known method for increasing solder joint height is to include "lifter pads" 30 beneath the non-solderable bottom surface of the SMD, as shown in FIG. 2. According to the prior art, these lifter pads are round in shape. When a solder mass 32 deposited on a lifter pad melts during reflow soldering, pressure within the molten solder mass provides an upward force F.sub.L which tends to lift the component, ideally maintaining it at or above a minimum solder joint height h.sub.o until the solder mass returns to a solid state.
While this method does tend to increase solder joint height, it makes no provision for the effect of the lifter pad on solder joint geometry. As the component is lifted by the lifter pads, solder quantity at both the inner and outer fillet areas is decreased as solder flows under the device termination area to fill in the increased solder height, thereby detrimentally altering solder joint geometry. Similarly, surface tension and pressure forces within the solder joint fillets detrimentally affect the shape and lifting effectiveness of the molten solder masses atop the lifter pads. Thus, using conventional lifter pads methods, inner and outer solder joint fillets tend to change shape and decrease in size, while lifter pad effectiveness in increasing solder joint height is reduced due to the effect of surface tension and pressure forces acting at the solder joints. Thus, while overall solder joint height may be increased, decreased overall crack propagation length and unfavorable interior fillet angles .alpha. may result, such that little or no overall improvement in crack initiation and propagation time is achieved.
Another approach for increasing solder joint height is disclosed in "Prediction of Equilibrium Shapes and Pedestal Heights of Solder Joints for Leadless Chip Components" (IEEE Transactions on Components, Packaging, and Mfg. Tech.), illustrated in FIG. 3. Rather than using lifter pads, this approach utilizes mounting pads whose outer edge 15 is spaced closer to the SMD than is the case for conventional mounting pads, so as to encourage the formation of convex inner and outer fillets, rather than the typical concave fillets. According to this approach, an amount of solder is deposited on the mounting pads sufficient to "float" the device on the solder to a desired solder joint height when subsequently reflowed.
This approach can be further illustrated by referring to FIGS. 4 and 5. FIG. 4 shows a free body diagram of a conventionally soldered (i.e., concave solder joint) SMD and the forces acting thereon during and after reflow. A solder joint height h.sub.o is achieved when the net downward force on the SMD (i.e., the weight W of the device, the ambient pressure P.sub.a, and the vertical components of the outer and inner fillet surface tension forces, F.sub.1 and F.sub.2, respectively) reaches equilibrium with the net upward force (i.e., the buoyancy and contact force p.sub.o provided by the solder joint). Compare this with FIG. 5, which shows a free body diagram of an SMD and the forces acting thereon during and after reflow, utilizing convex solder joints. Note that the vertical components of the surface tension forces F.sub.1 and F.sub.2 exert less downward force on the SMD than do the vertical components of F.sub.1 and F.sub.2 in FIG. 4. In addition, the convex shape in FIG. 5 guarantees that P.sub.o &gt;P.sub.a (i.e., a net upward pressure force), while in FIG. 4 the concave shape suggests that p.sub.a &gt;p.sub.o (i.e., a net downward pressure force), particularly for devices having relatively wide solder joints. Thus, by utilizing the convex joint geometry illustrated in FIG. 5, a greater solder height h.sub.o can be achieved than can be realized by using the conventional concave solder joint geometry.
However, in practice both of the above prior art methods are limited in their applicability by limits between design of the mounting or lifter pads and the quantity of solder which can be deposited thereon using the standard solder paste deposition process (i.e., screen printing). While other methods exist which can deposit additional solder paste (e.g., dispensing), these are generally much slower than screen printing and so are most practically employed at additional cost only when needed.
Furthermore, while both of these approaches may be effective in achieving a desired solder joint height, they unfortunately may have detrimental effects on solder joint life. For example, the prior art lifter pad method produces concave solder joints having a decreased distance between the inner fillet surface and the outer fillet surface (i.e., decreased overall crack propagation length). Also, while the prior art convex geometry method provides a solder joint height which would increase stage II crack propagation time somewhat, neither this method nor the prior art lifter pad method addresses how to appropriately design the inner fillet geometry, nor how to optimize the mounting pad design to maximize the outer fillet crack propagation distance.
It is desirable, therefore, to provide a method for improving solder joint durability by achieving increased solder joint height, optimized solder joint inner fillet shape, and optimized solder joint outer fillet size and shape.